This invention relates generally to a method for determining azimuthal direction and more particularly to a method for determining azimuthal direction relative to the earth's true north using gyroscopic apparatus.
As is known, gyroscopic apparatus have been used for determining azimuthal direction relative to true north where, for example, the accuracy attainable with a magnetic compass is inadequate. The use of such apparatus is based on the fact that, because the earth rotates about the true north-south axis, there will be no component of the earth's rotational rate about the east-west axis. Consequently, a gyroscope having an input axis maintained orthogonal to the earth's vertical axis together with mutually orthogonal spin axis and output axis may be used to determine the north-south axis. This is so because, absent any error within such gyroscope, the amount of precession at the output axis will be zero when the input axis is aligned with the east-west axis. In any practical application, however, there are developed within the gyroscope apparatus torques attributable to, inter alia, imperfections in gimbal bearings of the apparatus. Such torques result in precessional rates (i.e. drifts) at the output axis which are indistinguishable from precessional rates attributable to the earth's rotation. Because the amount of drift is generally unknown the accuracy in measuring azimuthal direction with such a gyroscope may, in some applications, be unacceptable.
One method for improving the azimuthal direction measuring accuracy using gyroscopic apparatus has been to initially align the input axis of such gyroscope along the east-west axis, the latter such axis being determined by detecting the position of the input axis where the precessional rate at the output axis is zero and then, after reversing the direction of spin motor drive of the gyroscope's rotor, to realign the position of the input axis so that the precessional rate at the output axis is again zero. Such method assumes that the drift of the gyroscope will not significantly change after the direction of spin of the rotor is reversed. It follows, then, that the effect of drift may therefore be cancelled by assuming that the mean direction of the two alignments of the input axis is coaxial with the true east-west axis. The reversal of spin direction, however, may, unless an extremely stable gyroscope is used, give rise to unwanted torques which cause the drift in the gyroscopic apparatus to change after reversal. The assumption on which such method is based may therefore be invalidated. A second method is sometimes used to determine azimuthal direction relative to true north is to initially align the input axis of the gyroscope along an estimate of the earth's true east-west axis and, while so aligned, to measure the precessional rate at the output axis after the gyroscope has "settled out" (i.e. reached a steady state condition). The input axis may then be realigned 180.degree. from the initial alignment and the precessional rate again measured after the gyroscope has settled out. Therefore, with the assumption that the drift of the gyroscopic apparatus does not change between the first and second measurement, such drift can be calculated and its effect compensated to arrive at a true indication of the precessional rate of the output axis attributable to the earth's rotation. Such method, however, requires, typically, a ten minute "settling time" at each alignment position in order to determine the azimuthal direction to an accuracy within a quarter of a degree. Consequently, a relatively expensive gyroscope with low drift within such settling time must be used. Also, the realignment of the input axis will, because of rotational accelerations exerted on the gyroscope by such realignment procedure, develop forces within the gyroscopic apparatus, which inter alia, result in concomitant changes in the drift of such apparatus. The assumption on which the mmeasurement accuracy method is based thereby is invalidated.